This is the original IBM PC form factor. The difference between the PC and XT is that the XT was equipped with a hard drive. Original power supplies were physically large, but were only 63.5 Watts.

The IBM Personal Computer XT, often shortened to the PC XT or simply XT, was IBM's successor to the original IBM PC. It was released as IBM product number 5160 on March 8, 1983, and was one of the first computers to come standard with a hard drive. It was based on essentially the same architecture as the original PC, with only incremental improvements; a new 16-bit bus architecture would follow in the AT. The system was largely intended for business users, and a corresponding 3270 PC featuring 3270 terminal emulation was released later in October 1983. XT stands for eXtended Technology.

The XT originally came with 128 kB of memory, a 360 kB double-sided 5 1/4" full-height floppy disk drive, and a 10 MB hard drive, a serial port, eight 8-bit ISA expansion slots, and an Intel 8088 microprocessor running at 4.77 MHz (with a socket for an 8087 math coprocessor); the operating system usually sold with it was PC-DOS 2.0. The eight slots were an increase over the five in the IBM PC, although three were taken up by the floppy drive adapter, the hard drive adapter, and the serial card. Later models came with 256 kB of memory standard, and eventually models with 640 kB and a 20 MB hard drive were sold.

The XT originally came only in a standard configuration with the hard disk. It was not until 1985 that a model without the hard drive became available. Other models came with two half-height floppy drives as well as the hard drive.

In 1986, the XT/286 (IBM 5162) with a 6 MHz Intel 80286 processor was introduced. This system actually turned out to be faster than the ATs of the time using 8 MHz 286 processors due to the fact that it had zero wait state RAM that could move data more quickly.

Like the original PC, the XT came with a BASIC interpreter in ROM. Since this interpreter was meant to be used with a cassette drive (which wasn't offered on the XT), the only ways to access it were by disconnecting the hard drive and leaving the floppy drive empty, using the BASICA program, included on a floppy disk, which added extensions for using the disk drives, or invoking a BIOS call manually using a debugger.

PC and XT keyboards are not compatible with more modern PCs (IBM AT or newer), even with DIN to PS/2 mini-DIN plug adapters, because PC/XT keyboards use different keyboard scan codes. An XT to AT signal adapter is needed to create compatibility with modern computers.

These are the computers that had two power plugs, P8 and P9. These motherboards have fewer connectors directly on the motherboard. Instead, these boards use dongles to connect through to the case rear. Originally the AT and Baby AT power supplies were gigantic. Most cases that have Baby AT motherboards paired them with LPX power supplies.

LPX was invented by Western Digital. One unique design of the LPX form factor was the need for riser cards for the expansion slots that would run parallel to the motherboard. Not a hard standard, components could not be interchanged. LPX power supplies were commonly used with AT systems as well.

There was never any official LPX specification, but the design normally used consisted of the main I/O ports mounted on the back of the motherboard (something adopted by the ATX form factor), and a riser card in the center of the motherboard, on which the PCI and ISA slots were mounted. The PSU was of the same type used in the AT form factor. Due to the lack of standardized specification, riser cards were seldom compatible from one motherboard design to another, much less one manufacturer to another. The specification was very popular in the early-mid 90's, and briefly displaced the AT form factor as the most commonly used. However, the release of the Pentium II in 1997 effectively consigned the format to death, as a good airflow was important in Pentium II systems, and LPX systems normally had a rather poor airflow. The introduction of the AGP format was another blow, as the design not only increased the pincount on riser cards, but it limited most cards to one AGP, one PCI and one ISA slot, which was too restrictive for most users. LPX was phased out around 1998. NLX was the intended successor, though many manufacturers chose MicroATX or proprietary motherboard formats instead.

The ATX (for Advanced Technology Extended) form factor was created by Intel in 1995. It was the first big change in computer case and motherboard design in many years. ATX overtook AT completely as the default form factor for new systems. ATX addressed many of the AT form factor's annoyances that had frustrated system builders. Other standards for smaller boards (including microATX, FlexATX and mini-ITX) usually keep the basic rear layout but reduce the size of the board and the number of expansion slot positions. In 2003, Intel announced the new BTX standard, intended as a replacement for ATX. As of February 2006 the ATX form factor remains the industry standard for do-it-yourselfers; BTX has however made inroads into pre-made systems, being adopted by computer makers like Dell, Gateway, and HP.

The official specifications were released by Intel in 1995, and have been revised numerous times since, the most recent being version 2.2[1], released in 2004.

A full size ATX board is 12" wide by 9.6" deep (305 mm x 244 mm). This allows many ATX form factor chassis to accept microATX boards as well.

microATX (also known as µATX) is a small PC motherboard standard, with a maximum size of 9.6×9.6" (244×244 mm) (the standard ATX size is 12×9.6" maximum). microATX boards with integrated graphics are often used by motherboard manufacturers as a basis for small form factor and media center PCs.

microATX motherboards can utilize CPUs from either Intel or AMD. microATX boards often utilize the same north and southbridges as full ATX boards, allowing them to use the same components. However, since microATX towers are a lot smaller than typical ATX towers, there are fewer I/O ports. For example, an ATX motherboard usually supports up to five PCI expansion slots, while microATX boards typically have only three (four being the maximum permitted by the specification). Due to this limitation external USB hard drives, expansion cards, CD burners etc. may be necessary depending on the number of peripherals required.

WTX (for Workstation Technology Extended) is a motherboard form factor introduced by Intel in 1998, for its use at high-end, multiprocessor, multiple-hard-disks server and workstations.

This form factor is geared specifically towards the needs of high-end systems, and includes specifications for a WTX power supply unit (PSU).

The WTX specification was created to standardize a new motherboard and chassis form factor, fix the relative processor location, and allow for high volume airflow through a portion of the chassis where the processors are positioned. This allows for standard form factor motherboards and chassis to be used to integrate processors with more demanding thermal management requirements.

Bigger than ATX, maximum WTX motherboard size is 14"x16.75" (35.56 x 42.54 cm). This is intended to provide more room in order to accommodate higher numbers of integrated components.

WTX computer cases are backwards compatible with ATX motherboards (but not viceversa), and sometimes come equipped with ATX power supplies.

BTX (for Balanced Technology Extended) is a form factor for PC motherboards, originally slated to be the replacement for the aging ATX motherboard form factor in late 2004 and early 2005. It has been designed to alleviate some of the issues that arose from using newer technologies (which often demand more power and create more heat) on motherboards compliant with the circa-1996 ATX specification. The ATX and BTX standards were both proposed by Intel. Intel's decision to refocus on low-power CPUs, after suffering scaling and thermal issues with the Pentium 4, has added some doubt to the future of the form factor. The first company to implement BTX was Gateway Inc, followed by Dell. Although not marketed as such, Apple Computer's Mac Pro utilizes the BTX design as well. However, future development of BTX retail products by Intel was cancelled in September 2006.

Enhancements:

Low-profile - With the push for ever-smaller systems, a redesigned backplane that shaves inches off height requirements is a benefit to system integrators and enterprises who use rack mounts or blade servers.

Thermal design - The BTX layout establishes a straighter path of airflow with fewer obstacles, resulting in better overall cooling capabilities. A distinct feature of BTX is the vertical mounting of the motherboard on the left-hand side. This results in the graphics card heatsink facing upwards, rather than in the direction of the adjacent expansion card.

Structural design - The BTX standard specifies different locations for hardware mounting points, thereby reducing latency between devices and also reduces the physical strain imposed on the motherboard by heat sinks, capacitors and other components dealing with electrical and thermal regulation. For example, the Northbridge and Southbridge chips are located near each other and to the hardware they control.

Small form factor (SFF) computers are used in reference to computers with an overall size that are smaller than the normal ATX motherboard based cases. While there is no clear definition of "small form factor" many believe it has to do with either the volume of the computer case or whether or not the case is limited to a microATX motherboard or smaller.

A typical SFF PC is similar to a shoebox or book size in dimensions with a fairly square front profile or small tower look. Expandability is very limited in some small form factor computers with most commercial offering only providing for one 3.5" and one or two 5.25" external drive bays. Standard heatsinks may not fit in a SFF computer, with some manufacturers opting for a bespoke cooling system, and generally these cases can only fit 2-4 hard drives. The recent boom of the "box type" cases has allowed for standard ATX PSUs to be used in SFF computers.

There are many different types of small form factor computers available today. This article will discuss the three main types but keep in mind that there are many other types that use mini-itx motherboards and use other micro technologies.